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Based on your code and description, I try with custom control's property bind to custom control and custom control's property bind to control which get from Toolbox. The latter is works fine. But the former give me a error message which is fimilar with yours
"Object of type 'System.Windows.Data.Binding' cannot be converted to type 'System.String'".

But when I modify the MyText property as DependencyProperty, it works fine. Please check if the ControlToFocusOn is DependencyProperty.